Simultaneous double sided scanning of documents has been known for some time. The amount of data which can be transferred from the scanner to the host computer at any given time is limited, usually to 65,535 bytes. This would include a 20 byte header which identifies document size, resolution, bytes per image line, total bytes of image data in the packet, image compression, and the like.
Previously, devices of this kind waited until the entire scan was complete before deciding which side's scan would be transmitted first. Alternatively, first one entire side and then the second side were sent. From a performance standpoint, this resulted in a loss of efficiency, since the data was not being sent at or near the maximum permitted rate at all times.